1. Field of the Invention
The present invention relates to an apparatus for measuring the flow rate of a flowing medium.
2. Description of the Prior Art
Such apparatuses measure the flow rate of gases or liquids on the basis of a number of various principles. One type of flowmeter utilizes the circumstance that a flowing medium cools a heated body to an extent which, among other things, depends upon the rate of flow of the medium.
One such prior art flowmeter utilizes two thermistors which are both located in the flowing medium but of which only one is heated. By determining the difference between the resistance values of the two thermistors by means of, for example, a bridge connection, it has been possible to measure the flow rate. The disadvantage inherent in this measurement method is that it places very high demands on the mutual adaptation or matching of the thermistors. A very small difference between their resistance-temperature characteristics may easily give rise to errors of substantially the same magnitude as the magnitude of the measurement value itself. The very stringent tolerances involved entail that flowmeters of this type are extremely expensive.
Other prior art flowmeters (see, for example, U.S. Pat. No. 3,030,806) of the above-mentioned type utilize thermocouples, that is to say two conductors consisting of different metals which are soldered together at two points, whereby a thermoelectromotive force is generated if the soldered points are kept at different temperatures. If both of the soldered junctions are placed in the flowing medium and one of them is heated by means of an exterior source of heat, the voltage generated by the thermocouple will be a measurement of the rate of flow of the medium. The disadvantage of this type of flowmeter is the relatively low sensitivity of the thermocouple (the generated voltage is of the order of magnitude of 40 .mu.V/.degree.C. using copper constantan).